Hot Deformation Behavior and Isothermal Forging/Diffusion Bonding of Powder Metallurgy Tial Based Alloy

Author:Shi Cheng Cheng

Supervisor:zhang kai feng jiang shao song


Degree Year:2019





TiAl-based alloy is one of the promising light-weight materials serving for high temperature structural applications.In the aerospace industry,TiAl-based alloy components are able to boost power-mass characteristics,enabling weight loss and increase in speed limit as well as displacement limit.However,owing to its poor room temperature ductility,thermal workability and weldability,the industrial applications of TiAl-based alloys are still limited.Grain refinement has advantages such as enhancing room temperature ductility,improving thermal workability and broadening hot working window.In this research,pre-alloyed powders were used as starting materials.TiAl-based alloys with refined microstructure were fabricated by hot pressing sintering(HPS)and spark plasma sintering(SPS),respectively.Mechanical properties and high temperature fracture mechanism were investigated.Forged alloys with different microstructure were fabricated via pulse current assisted forging.Besides,mechanical properties have been improved.Pulse current assisted diffusion bonding was employed for alloy joining.TiAl-based hollow structure components were fabricated via isothermal forging combined with pulse current assisted diffusion bonding.Pre-alloyed Ti46.5Al2Cr1.8Nb0.2W0.15B powders were applied to prepare TiAl-based alloys through HPS.The effects of sintering parameters on microstructure evolution and mechanical properties were investigated in detail.It was found thatα2→γphase transformation occur during the sintering process.Partial dynamic recrystallization happened.When sintering temperature was in the range of 1100-1300°C,the alloy was composed of refined nearγmircrostructure.With increasing sintering temperature to 1350°C,coarse fully lamellar microstructure formed.In addition,it was found that high temperature mechanical properties of the sintered alloys were remarkably influenced by sintering temperature and holding time.As the sintering temperature or holding time increased,high temperature ductility of the sintered alloys increased firstly and then decreased when tensile tested at 800°C.The elongation of 1300°C/120min sintered alloy reached as high as 80%.This was caused by the gradual elimination of the original powder boundaries and the heterogeneous microstructures when the sintering temperature and holding time increased.Because of that,the fracture mechanism changed from fracture at original powder boundaries to micro-pore aggregation and growth fracture.TiAl-based alloys with nearγmicrostructure,duplex microstructure,near lamellar microstructure and fully lamellar structure were fabricated by SPS.Heterogeneous microstructure and original powder boundaries could be observed in all of the SPS sintered alloys.High temperature tensile testing showed fracture at the original powder boundaries and the heterogeneous microstructure compromised their high temperature mechanical properties.High temperature deformation behavior inα+γregion of nearγTiAl based alloys were investigated through isothermal compression tests.Constitutive equation and hot processing map were established.Combined with microstructure observation,the instability zone of deformation was determined:temperature range 1125°C-1155°C,strain rate range 0.1s-1-6.3×10-2s-1.It was found that recrystallization and phase transformation occur during the hot deformation process.Dynamic recrystallization model was established to investigate microstructure evolution.The model showed deformation twins played an important role in low temperature deformation.And they disappeared during the recrystallization process.Besides,microstructure changed from nearγmicrostructure to duplex microstructure as temperature increased.Based on the investigation on hot deformation behavior,forged TiAl-based alloys were prepared via pulse current assisted isothermal forging.Compared to the sintered alloys,the elongation at 800°C increased to 110%.Furthermore,when tensile tested at1000°C/2×10-4s-1,the elongation of the forged alloy achieved above 410%,showing excellent superplastic property.Through isothermal compression test,high temperature deformation behavior of TiAl-based alloys with duplex microstructure was investigated.The constitutive equation and hot processing map were established,and microstructure evolution were studied.It was found that dynamic recrystallization happened in the initialγphase.Furthermore,initial lamellar structures gradually decomposed in the manner of dynamic recrystallization.With temperature increasing,initial lamellar structures were decomposed completely and replaced by newly formed lamellar structure.Alloys with nearγmicrostructure and duplex microstructure were used to prepare forged TiAl-based alloys.It was found nearγmicrostructure,duplex microstrucgture and near lamellar microstructure formed in an order as temperature increased.After forging,original powder boundaries and heterogeneous microstructures were eliminated.High temperature tensile testing showed high temperature ductility were improved remarkably from 45%to 106%.This is due to fracture mechanism changes from original powder boundary fracture to micro-pore aggregation and growth.Bonding of TiAl-based alloys were realized via pulse current assisted diffusion bonding.The microstructure evolution of interfacial microstructure and mechanical properties was analyzed.It was found that shear strength could reach above 90%of substrate materials for diffusion parameters of 1100°C/20min/25MPa while the bonding efficiency increases remarkably compared to hot pressing bonding.Based on above findings,typical hollow structures were manufactured via isothermal forging and pulse current assisted diffusion bonding.This research might inspire the application of TiAl-based alloys.